1. Field of the Invention
The present invention is in the field of telescopes which can be used both in the day time to obtain a magnified view of a distant scene, and which can also be used at night or under other low-light conditions to obtain a view of the distant scene which is both magnified and intensified or amplified to provide a visible image of a scene too dark to be viewed with natural vision. The present invention also relates to such telescopes which are equipped with a reticle for use in sighting a weapon in both day-light and low-light conditions.
2. Related Technology
A conventional day/night telescopic sight is known in accord with U.S. Pat. No. 5,084,780, issued Jan. 18, 1992 to E. A. Phillips. The Phillips patent appears to teach a telescopic day/night sight which has several alternative embodiments. According to one embodiment set out in the Phillips patent, such a telescopic sight includes an objective lens behind which is disposed an angulated dichroic mirror. This mirror divides light coming into the sight via the objective lens into two frequency bands. Light of longer wavelengths (lower frequencies) is allowed to pass through the dichroic mirror to an image intensifier tube. This image intensifier tube operates in the conventional way familiar to those ordinarily knowledgeable about night vision devices. That is, the image intensifier tube provides a visible image which replicates a dim image or an image formed by invisible infrared light. Thus, the longer wavelength band which passed through the dichroic mirror includes the infrared portion of the spectrum, and provides to the image intensifier tube the frequencies of light to which the tube is most responsive. The visible portion of the light entering the sight via the objective lens is reflected by the dichroic mirror into an optical system leading to a combiner and to an eyepiece. At the combiner, the image provided by the image intensifier tube is superimposed on the image from the visible-light channel of the sight, and the resulting combined image is presented to a user of the sight via the eyepiece.
A possible disadvantage of the Phillips sight as described above is that the angulated dichroic mirror can introduce both parallax, astigmatism, and color aberrations into the image provided to the user. Thus, slight movements of the sight may cause the user to experience some shifting of the image along a line parallel with the angulation of the mirror, while the image does not shift along a line perpendicular to this angulation. In other words such an angulated dichroic mirror may result it the slight jiggling inherent in a hang-held telescope or weapon sight amplifying the apparent movement of the image in at least one direction. This effect can be disconcerting for the user of the device.
Other versions of the Phillips sight use a separate objective lens for both the day channel and the night channel of the sight. These versions would not appear to suffer from the same possible parallax problem described above with respect to the versions using the dichroic mirror. However, the versions of Phillips sight with two objective lenses suffer from an increased size, weight, and expense because of the additional optics and larger housing required to mount and protect these optics.
In each case with the sight of Phillips, the optical channels for the night sight and the day sight are laterally offset relative to one another. These two offset optical channels are parallel, and the image from these channels is combined for presentation at the eyepiece. However, in each case, the sight taught by Phillips requires separate laterally offset optical channels, and presents the problem of correctly and precisely superimposing the image from these two channels for the user of the sight.